The amount of money being poured into the mayor’s race has topped $6 million, a sum that is fueled by new campaign finance rules that give special interests and wealthy donors a much larger influence on city elections than in years past.

The four major candidates — City Councilman Carl DeMaio, District Attorney Bonnie Dumanis, Rep. Bob Filner and Assemblyman Nathan Fletcher — have amassed nearly $4.1 million in campaign contributions through May 19, the end of the last reporting period. Political-action committees backing or opposing the candidates have raised nearly $1.3 million. Political parties, unions and other groups have spent at least $660,000 on top of that.

John Dadian, a Republican political consultant not involved in the race, said he’s not surprised by the amount of spending, but the tone of the advertising gives him pause.

“It tells me that it’s a very competitive race and when it’s that competitive you need to be spending dough to get your message out,” he said. “Usually in the past you saw independent expenditures in favor of somebody. I’ve never seen the explosion of several independent expenditures that are specifically targeted against somebody.”

Independent expenditure is the technical term for money spent on the race that isn’t coordinated with any particular candidate. The resulting ads are often more negative because the money trail doesn’t lead back to the candidate but rather a financial backer.

Most of the spending in the race revolves around DeMaio and Fletcher. They’ve each raised far more than their rivals and multiple PACs have formed to support or oppose them.

Among the candidates, DeMaio leads overall with nearly $1.7 million raised, although he has contributed $675,000 of his own money to that total and the local Republican Party gave him $65,000. Fletcher has the most money from individual donors, with nearly $1.3 million raised, including about $303,000 in the last period, the most of any candidate.

Dumanis and Filner continued to trail in fundraising. Dumanis raised about $50,000 from donors and then loaned her campaign an additional $40,000, bringing her total campaign haul to $677,000. Filner collected about $92,000 from donors for a total of $372,000, but he also received an additional $67,000 from the local Democratic Party.

Many political consultants predicted a spending spree for this mayor’s race in large part because of a court case that struck down several of the city’s campaign-finance laws.

The floodgates opened in January 2010 when the U.S. Supreme Court ruled the federal government may not ban political spending by corporations and other advocacy groups, such as labor unions. The local Republican Party and others had already filed a lawsuit against the city challenging many of the contribution limits in place when the Citizens United decision landed. Rulings in the San Diego case over the past two years upheld some rules and obliterated others.